The concept of additive manufacturing has been developed in various directions, namely 3D-printing, 3D-prototyping, and rapid manufacturing. These technologies are widely used to produce a broad range of parts in various industries. Unlike conventional machining technologies, where material is removed from plain solid blocks to obtain the desired part, additive manufacturing technologies produce the solid body layer by layer by super-positioning cross sections of the raw material. Typically, custom-made products are manufactured directly from its 3D computer aided design (CAD) model, which is pre-processed —for part orientation, creation of horizontal cross sections, and definition of other relevant process parameters—and fed as an input file to the manufacturing machine control. Additive manufacturing is capable also of making parts that otherwise would be impossible to produce with conventional machining methods, or require expensive molds or dies. Currently, the technologies used for direct manufacturing of metallic parts from powders are selective laser sintering (or melting), stereo-lithography, direct-metal laser sintering and spark-plasma sintering. Such devices are generally too large and costly to be widely used.
US2004232583 describes an additive fabrication process for sequentially incrementing a workpiece to form a desired object, the process comprising:
a. providing a layer of pulverulent substrate;
b. selectively applying at least one microwave-absorbing susceptor to one or more regions of the substrate;
c. treating the layer at least once with microwave radiation, to melt the regions containing the susceptor.
d. cooling the layer.
GB2422344 describes an additive fabrication process for sequentially incrementing a workpiece to form a desired object, the process comprising the same steps a, b and c described in US2004232583, but using infrared radiation to meld the powder.
The cited prior art patents requires extending a layer of source material, and adding a susceptor to some regions of said source material layer. It has to be noted though that US2004232583 does not employ a localized microwave applicator, and that it requires adding susceptors (microwave absorbing additives) to the raw material, hence this method is not applicable for instance to pure metal powders. Regarding GB2422344, infrared is a different type of radiation than microwaves. It is generated and guided by completely different devices, and has different physical properties.
U.S. Pat. No. 6,243,616 B1 describes an additive fabrication device for sequentially incrementing a workpiece to form a desired object, the device comprising:                a feeder for providing source material onto a surface in a desired uniform thickness;        a microwave apparatus including a microwave generator able to create a microwave radiation in the frequency range of 430 to 6800 MHZ, and a microwave applicator able to provide said microwave, preferably focused onto a beam width of 0.1 to 3 mm, particularly preferably 0.3 to 1 mm;        a controlled displacement mechanism for generate relative displacement between said support structure and said microwave applicator so that a layer by layer structure is performed.        
The referenced patent describes a microwave frequency range focused in a beam with a size which is essentially larger than the wavelength, so it can't induce a thermal-runaway process (note that U.S. Pat. No. 6,243,616 B1 does not present any implementation of the microwave applicator that can focus the microwave energy to a spot smaller than a wavelength, and it does not present any physical mechanism that may compensate for the natural diffraction of the microwave energy expected in the schemes presented there). Further this document deals also with a layer-by-layer deposition, whereas we propose, as discussed below, also localized feeding of the raw material either in a powder or a wire form, directly by and/or in combination with a microwave applicator. U.S. Pat. No. 6,243,616 B1 is relevant mainly for polymer materials, and to microwave absorbing materials (see column 4, lines 46-47, “. . . (materials) have a dipolar basic structure”, hence it is not applicable for pure metal powders (which obviously have no dipolar structure). The irrelevance to pure metals is also indicated clearly in the description of the invention there (see column 4, Line 49, where the applicability is limited to “metal powders coated with these polymers”, whereas the latter are needed as microwave absorbers. In the present invention, other mechanisms are employed, such as localized microwave inner plasma breakdown; hence the present invention is applicable also for metal powders, with no coatings or additives as the previous cited prior arts.
U.S. Pat. No. 6,214,279 describes an additive fabrication process, which deals with composite materials and multi-component fabrication, using an adhesive product to solidify the source material.
With regard to the previous art the proposed invention provide an additive fabrication method and corresponding device in which localized microwaves are employed to melt or sinter the source material to increment the object being fabricated wherein localized microwave radiation is applied through a near-field microwave applicator configured to apply the microwave radiation to transform the source material within an included volume receiving the localized microwave radiation, said included volume having at least one dimension smaller than a wavelength of the microwave radiation induces intentionally either a thermal-runaway instability and/or inner plasma breakdown in between the source material particles.
U.S. Pat. No. 6,114,676 discloses, but in the context of a microwave drill technology, localized application of microwaves” preferably achieved by use of a near-field applicator, to eliminate material from a solid concentrating the microwave radiation into a volume having at least one dimension smaller than the microwave wavelength, and most preferably smaller than half the microwave wavelength, thereby inducing a thermal-runaway process in which the local temperature increase is accelerated by the temperature-dependent parameters of the raw material, and a confined hotspot is formed rapidly. According to this method, small amounts of source material in front of the applicator are melted and removed. Therefore, it has to be noted that while the present invention employs a similar applicator to U.S. Pat. No. 6,114,676, the purpose is totally different, as the present invention deals with additive construction, mainly of powders, whereas U.S. Pat. No. 6,114,676 deals mostly with removing (e.g. drilling) of solid materials. The joining application in U.S. Pat. No. 6,114,676 refers to metal parts rather than powders.